The laser Doppler: a non-invasive measure of cochlear blood flow

Usefulness of Intravital Multiphoton Microscopy in Visualizing Study of Mouse Cochlea and Volume Changes in the Scala Media

Conventional microscopy has limitations in viewing the cochlear microstructures due to three-dimensional spiral structure and the overlying bone. But these issues can be overcome by imaging the cochlea in vitro with intravital multiphoton microscopy (MPM). By using near-infrared lasers for multiphoton excitation, intravital MPM can detect endogenous fluorescence and second harmonic generation of tissues. In this study, we used intravital MPM to visualize various cochlear microstructures without any staining and non-invasively analyze the volume changes of the scala media (SM) without removing the overlying cochlear bone. The intravital MPM images revealed various tissue types, ranging from thin membranes to dense bone, as well as the spiral ganglion beneath the cochlear bone. The two-dimensional, cross-sectional, and serial z-stack intravital MPM images also revealed the spatial dilation of the SM in the temporal bone of pendrin-deficient mice. These findings suggest that intravital MPM might serve as a new method for obtaining microanatomical information regarding the cochlea, similar to standard histopathological analyses in the animal study for the cochlea. Given the capability of intravital MPM for detecting an increase in the volume of the SM in pendrin-deficient mice, it might be a promising new tool for assessing the pathophysiology of hearing loss in the future.

Two-photon microscopy allows imaging and characterization of cochlear microvasculature in vivo

Impairment of cochlear blood flow has been discussed as factor in the pathophysiology of various inner ear disorders. However, the microscopic study of cochlear microcirculation is limited due to small scale and anatomical constraints. Here, two-photon fluorescence microscopy is applied to visualize cochlear microvessels. Guinea pigs were injected with Fluorescein isothiocyanate- or Texas red-dextrane as plasma marker. Intravital microscopy was performed in four animals and explanted cochleae from four animals were studied. The vascular architecture of the cochlea was visualized up to a depth of 90.0±22.7 μm. Imaging yielded a mean contrast-to-noise ratio (CNR) of 3.3±1.7. Mean diameter in vivo was 16.5±6.0 μm for arterioles and 8.0±2.4 μm for capillaries. In explanted cochleae, the diameter of radiating arterioles and capillaries was measured with 12.2±1.6 μm and 6.6±1.0 μm, respectively. The difference between capillaries and arterioles was statistically significant in both experimental setups (P<0.001 and P=0.022, two-way ANOVA). Measured vessel diameters in vivo and ex vivo were in agreement with published data. We conclude that two-photon fluorescence microscopy allows the investigation of cochlear microvessels and is potentially a valuable tool for inner ear research.

Changes in cochlear blood flow in mice due to loud sound exposure measured with Doppler optical microangiography and laser Doppler flowmetry

In this work we determined the contributions of loud sound exposure (LSE) on cochlear blood flow (CoBF) in an in vivo anesthetized mouse model. A broadband noise system (20 kHz bandwidth) with an intensity of 119 dB SPL, was used for a period of one hour to produce a loud sound stimulus. Two techniques were used to study the changes in blood flow, a Doppler optical microangiography (DOMAG) system; which can measure the blood flow within individual cochlear vessels, and a laser Doppler flowmetry (LDF) system; which averages the blood flow within a volume (a hemisphere of ~1.5 mm radius) of tissue. Both systems determined that the blood flow within the cochlea is reduced due to the LSE stimulation.

Betahistine exerts a dose-dependent effect on cochlear stria vascularis blood flow in guinea pigs in vivo

OBJECTIVE

Betahistine is a histamine H(1)-receptor agonist and H(3)-receptor antagonist that is administered to treat Menière's disease. Despite widespread use, its pharmacological mode of action has not been entirely elucidated. This study investigated the effect of betahistine on guinea pigs at dosages corresponding to clinically used doses for cochlear microcirculation.

METHODS

Thirty healthy Dunkin-Hartley guinea pigs were randomly assigned to five groups to receive betahistine dihydrochloride in a dose of 1,000 mg/kg b. w. (milligram per kilogram body weight), 0.100 mg/kg b. w., 0.010 mg/kg b. w., 0.001 mg/kg b. w. in NaCl 0.9% or NaCl 0.9% alone as placebo. Cochlear blood flow and mean arterial pressure were continuously monitored by intravital fluorescence microscopy and invasive blood pressure measurements 3 minutes before and 15 minutes after administration of betahistine.

RESULTS

When betahistine was administered in a dose of 1.000 mg/kg b. w. cochlear blood flow was increased to a peak value of 1.340 arbitrary units (SD: 0.246; range: 0.933-1.546 arb. units) compared to baseline (p<0.05; Two Way Repeated Measures ANOVA/Bonferroni t-test). The lowest dosage of 0.001 mg/kg b. w. betahistine or NaCl 0.9% had the same effect as placebo. Nonlinear regression revealed that there was a sigmoid correlation between increase in blood flow and dosages.

CONCLUSIONS

Betahistine has a dose-dependent effect on the increase of blood flow in cochlear capillaries. The effects of the dosage range of betahistine on cochlear microcirculation corresponded well to clinically used single dosages to treat Menière's disease. Our data suggest that the improved effects of higher doses of betahistine in the treatment of Menière's disease might be due to a corresponding increase of cochlear blood flow.

Volumetric Imaging of Blood Flow within Cochlea in Gerbil in vivo

Changes in blood flow to the inner ear are thought to influence a number of cochlear diseases, including noise-induced hearing loss, sudden hearing loss, and Meniere's disease. Advances have been made in the areas of vital microscopic studies of micro-circulation, and the laser Doppler flowmetry. But none of these techniques can provide in vivo three-dimensional (3-D) mapping of microvascular perfusion within the cochlea. To overcome this limitation we have developed and used a method of optical microangiography (OMAG) that can generate 3-D angiograms within millimeter of tissue depths by analyzing the endogenous optical scattering signal obtained from an illuminated sample. We used OMAG to visualize the cochlear microcirculation of adult living gerbil through the intact cochlea, which would be difficult, if not impossible, by use of any other current techniques.

An animal model for the analysis of cochlear blood flow [corrected] disturbance and hearing threshold in vivo

Impairment of cochlear blood flow (CBF) is considered to be important in inner ear pathology. However, direct measurement of CBF is difficult and has not been investigated in combination with hearing function. Six guinea pigs were used to show feasibility of an animal model for the analysis of cochlear microcirculation by intravital microscopy in combination with investigation of the hearing threshold by brainstem response audiometry (ABR). By the application of sodium nitroprusside (SNP), CBF was increased over 30 min. Reproducibility of measurements was shown by retest measurements. Mean baseline velocity of CBF was 109 +/- 19 mum/s. Vessel diameters had a mean value of 9.4 +/- 2.7 mum. Mean hearing threshold was 19 +/- 6 dB. In response to SNP, CBF velocity increased significantly to 161 +/- 26 mum/s. Mean arterial pressure decreased significantly to 36 +/- 11 mmHg. After the end of the application, CBF velocity recovered to a minimum of 123 +/- 17 microm/s. Within the retest, CBF velocity significantly increased to a maximum of 160 +/- 31 microm/s. Second recovery of CBF velocity was 125 +/- 14 mum/s. Within the second retest, CBF increased significantly to 157 +/- 25 microm/s. ABR thresholds did not change significantly. The increase in blood flow velocity occurred in spite of substantial hypotension as induced by a vasodilator. This may explain the fact that ABR threshold remained unchanged reflecting a maintained blood supply in this part of the brain. This technique can be used to evaluate effects of treatments aimed at cochlear microcirculation in inner ear pathologies.

In vivo imaging of mammalian cochlear blood flow using fluorescence microendoscopy

AIMS

We sought to develop techniques for visualizing cochlear blood flow in live mammalian subjects using fluorescence microendoscopy.

BACKGROUND

Inner ear microcirculation appears to be intimately involved in cochlear function. Blood velocity measurements suggest that intense sounds can alter cochlear blood flow. Disruption of cochlear blood flow may be a significant cause of hearing impairment, including sudden sensorineural hearing loss. However, inability to image cochlear blood flow in a nondestructive manner has limited investigation of the role of inner ear microcirculation in hearing function. Present techniques for imaging cochlear microcirculation using intravital light microscopy involve extensive perturbations to cochlear structure, precluding application in human patients. The few previous endoscopy studies of the cochlea have suffered from optical resolution insufficient for visualizing cochlear microvasculature. Fluorescence microendoscopy is an emerging minimally invasive imaging modality that provides micron-scale resolution in tissues inaccessible to light microscopy. In this article, we describe the use of fluorescence microendoscopy in live guinea pigs to image capillary blood flow and movements of individual red blood cells within the basal turn of the cochlea.

METHODS

We anesthetized eight adult guinea pigs and accessed the inner ear through the mastoid bulla. After intravenous injection of fluorescein dye, we made a limited cochleostomy and introduced a compound doublet gradient refractive index endoscope probe 1 mm in diameter into the inner ear. We then imaged cochlear blood flow within individual vessels in an epifluorescence configuration using one-photon fluorescence microendoscopy.

RESULTS

We observed single red blood cells passing through individual capillaries in several cochlear structures, including the round window membrane, spiral ligament, osseous spiral lamina, and basilar membrane. Blood flow velocities within inner ear capillaries varied widely, with observed speeds reaching up to approximately 500 microm/s.

CONCLUSION

Fluorescence microendoscopy permits visualization of cochlear microcirculation with micron-scale optical resolution and determination of blood flow velocities through analysis of video sequences.

Cochlear blood flow modifications induced by anaesthetic drugs in middle ear surgery: comparison between sevoflurane and propofol

OBJECTIVE

Because it is necessary to maintain controlled hypotension during middle ear surgery in order to avoid bleeding and as it is known that cochlear blood flow (CBF) is related to blood pressure (BP), it is useful to evaluate CBF modifications induced by anaesthetics in order to prevent cochlear damage. The aim of this paper is to evaluate, using laser Doppler flowmetry, which anaesthetic drug, out of sevoflurane and propofol, has the smallest effect on CBF.

MATERIAL AND METHODS

Twenty consenting adult patients scheduled for myringoplasty under general anaesthesia for simple tympanic membrane perforation were studied. Patients were divided into two groups: the first group was treated with sevoflurane and the second with propofol. For the first group, CBF measurement was carried out on three different occasions: (i) at a basal low drug dosage; (ii) having increased the drug dosage to a higher level; and (iii) having reduced the drug dosage to the basal low level again. For the second group, CBF measurement was carried out on three different occasions: (i) 10 min after injecting a bolus of propofol; (ii) immediately after a second propofol injection; and (iii) 10 min after a third injection of propofol. A probe was placed over the promontory in order to measure CBF levels.

RESULTS

In the subjects treated with sevoflurane, after having increased the drug dosage, BP decreased significantly while CBF did not change significantly. In the subjects treated with propofol we recorded a significant reduction in BP, as well as a decrease in CBF.

CONCLUSION

The results obtained show that sevoflurane has a hypotensive effect without modifying CBF, while propofol, although having a similar effect on BP to sevoflurane, has less of a protective effect on inner ear microcirculation.

A role for the angiotensin IV/AT4 system in mediating natriuresis in the rat

Angiotensin II (AngII) or Angiotensin IV (AngIV) was infused into the renal artery of anesthetized rats while renal cortical blood flow was measured via laser Doppler flowmetry. The infusion of AngII produced a significant elevation in mean arterial pressure (MAP) with an accompanying decrease in cortical blood flow, glomerular filtration rate (GFR), urine volume, and urine sodium excretion. The infusion of AngIV induced significant increases in renal cortical blood flow and urine sodium excretion, without altering MAP, GFR, and urine volume. Pretreatment infusion with a specific AT1 receptor antagonist, DuP 753, blocked or attenuated the subsequent AngII effects, while pretreatment infusion with the specific AT4 receptor antagonist, Divalinal-AngIV, blocked the AngIV effects. These results support distinct and opposite roles for AngII and AngIV, i.e. AngII acts as an anti-natriuretic agent, while AngIV acts as a natriuretic agent.

Bone conduction experiments in animals - evidence for a non-osseous mechanism

Bone conducted stimuli are used to differentiate between conductive and sensori-neural hearing loss. It has been thought that the main route for the transfer of vibratory energy from the point of application of the bone vibrator on the skull to the inner ear is completely osseous. An additional mechanism may play a prominent role. In rats, a bone vibrator was applied to the skull and also directly on the brain, after removing bone (a craniotomy), exposing the brain. Auditory nerve-brainstem evoked response (ABR) could be elicited not only with the vibrator on bone, but also with the vibrator directly on the brain. Similar results were obtained in guinea-pigs and fat sand rats. Noise masked this ABR. Extensive removal of skull bone did not alter the ABR to bone-conducted stimuli delivered to the exposed brain. Experimental elimination of the ossicular chain inertial mechanism and of the occlusion effect did not greatly alter the bone conduction response. A reduction in the fluid volume of the cranial cavity induced threshold elevations of the bone conducted ABR but not of the air conducted ABR. These findings can be interpreted as evidence that the 'classical' bone conduction mechanisms should be modified to include a major pathway for cochlear excitation which is non-osseous: when a bone vibrator is applied to the skull, the bone vibrations may induce audio-frequency sound pressures in the skull contents (brain and cerebro-spinal fluid) which are then communicated by fluid channels to the fluids of the inner ear.

The optical properties of the cochlear bone

The transmittance, reflectance and the angular dependence of coherent light at wavelengths of 632.8 and 750 nm in the cochlear bone in guinea pigs were measured by a single integrating sphere and a goniometer. The strict one-dimensional transport theory was applied to calculate the absorption and scattering coefficients. The Henyey-Greenstein phase function was applied to determine the anisotropy factor g, from the goniometer measurements. A linear inverse correlation was found between the calculated g and sample thickness. We found that the cochlear bone in guinea pigs strongly scatters light in the forward direction. Its absorption and scattering coefficients are much larger than those of dermis. Delrin has a similar optical absorption property to human dermis tissue.

The tonic sympathetic input to the cochlear vasculature in guinea pig

Vascular tones is an essential component in maintaining steady regional blood flow and dynamic responsiveness of a vascular bed. Sympathetic innervation can contribute to vascular tone. Although certain studies have reported evoked changes in cochlear blood flow (CBF) with activation of the sympathetic fibers to the cochlear vasculature, other studies have failed to show evidence of sympathetic contribution to CBF regulation when the cervical sympathetic fibers were unilaterally sectioned. We hypothesized that the bilateral 'sympathectomy of the stellate ganglia' would remove sufficient sympathetic input to the cochlea to yield a change in CBF resting level. To test this hypothesis a new technique was used to expose the stellate ganglia (SG) bilaterally and induce a chemical sympathectomy. We observed that unilateral SG blockade with 2 microliters of 4 mM lidocaine hydrochloride on either side produced a 5-10% increase in CBF, which recovered to baseline during the following 2 min. A subsequent blockade of the contralateral SG produced a rapid 25-35% increase, which then recovered partially during the following 3-4 min, remaining 5-15% above the baseline over a 20 min measurement period. Superior cervical ganglion transection did not affect CBF. Our results provide evidence for the existence of a tonic sympathetic component in the control of vascular tone in guinea pig cochlea. This neural effect is derived bilaterally from SG. This result is consistent with previous anatomical studies showing the bilateral innervation of the cochlea by the SG sympathetic fibers and with previous physiological studies on the bilaterality of evoked changes in CBF due to electric stimulation of SG.

Effect of leukotriene inhibitor on cochlear blood flow in salicylate ototoxicity

Our previous studies showed that salicylate ototoxicity is associated with decreased levels of vasodilating prostaglandins (PGs) and increased vasoconstricting leukotrienes (LTs) in the perilymph and reduced cochlear blood flow (CoBF). The purpose of this study was to test the hypothesis that leukotriene inhibitor prevents salicylate ototoxicity by preventing abnormal elevation of LT levels in the inner ear, thus averting a decrease in CoBF resulting from abnormal levels of arachidonic acid metabolites in the inner ear. Ototoxicity was induced in chinchillas by either local round window membrane (RWM) application or systemic treatment with salicylate both with and without pretreatment with leukotriene inhibitor (Sch 37224). A moderate reduction in CoBF was documented with both local RWM and systemic treatment with salicylate. Salicylate induced hearing loss and reduction in CoBF were prevented by pretreatment with a leukotriene inhibitor. This study suggests that leukotriene inhibitor prevents salicylate ototoxicity by averting a decrease in CoBF mediated by abnormal levels of arachidonic acid metabolites in the inner ear.

Brain angiotensin receptor subtypes in the control of physiological and behavioral responses

This review summarizes emerging evidence that supports the notion of a separate brain renin-angiotensin system (RAS) complete with the necessary precursors and enzymes for the formation and degradation of biologically active forms of angiotensins, and several binding subtypes that may mediate their diverse functions. Of these subtypes the most is known about the AT1 site which preferentially binds angiotensin II (AII) and angiotensin III (AIII). The AT1 site appears to mediate the classic angiotensin responses concerned with body water balance and the maintenance of blood pressure. Less is known about the AT2 site which also binds AII and AIII and may play a role in vascular growth. Recently, an AT3 site was discovered in cultured neoblastoma cells, and an AT4 site which preferentially binds AII(3-8), a fragment of AII now referred to as angiotensin IV (AIV). The AT4 site has been implicated in memory acquisition and retrieval, and the regulation of blood flow. In addition to the more well-studied functions of the brain RAS, we review additional less well investigated responses including regulation of cellular function, the modulation of sensory and motor systems, long term potentiation, and stress related mechanisms. Although the receptor subtypes responsible for mediating these physiologies and behaviors have not been definitively identified research efforts are ongoing. We also suggest potential contributions by the RAS to clinically relevant syndromes such as dysfunctions in the regulation of blood flow and ischemia, changes in cognitive affect and memory in clinical depressed and Alzheimer's patients, and angiotensin's contribution to alcohol consumption.

Effects of posture, hypotension and locally applied vasoconstriction on the middle ear microcirculation in anaesthetized humans

Studies by laser-Doppler flowmetry of middle ear microcirculation changes induced by physical and chemical stimuli in the animal have only recently been made. This prospective study, performed in humans, was designed to compare the effects of a postural manoeuvre (headup tilt 30 degrees), hypotension and locally applied vasoconstriction on middle ear blood flow during anaesthesia. Circulatory changes provoked by a headup tilt of 30 degrees, and successive intravenous boluses of potent vasodilators, were compared with circulatory changes provoked by locally applied adrenaline, in ten healthy patients in good physical states undergoing middle ear surgical repair. Heart rate and direct arterial pressure were continuously recorded via a radial artery cannula. Middle ear blood flow was continuously recorded via a laser-Doppler probe placed on the promontorium cavi tympani. Metabolic parameters (partial pressure of O2 and CO2 in arterial blood, pH, arterial lactate concentrations) and arterial concentrations of propofol were measured just before and just after the experiment. Headup tilt did not modify heart rate, mean arterial pressure or middle ear blood flow. Vasodilators (nicardipine, nitroprusside, nitroglycerin) provoked a fall in arterial pressure (P < 0.0001, P < 0.0001, P < 0.019, respectively), but did not induce any significant variations in heart rate; variations occurred in middle ear blood flow (P > 0.05, not significant) which were different according to patients and agents. Locally applied adrenaline provoked a fall in the middle ear blood flow (P < 0.0012), with no effect on heart rate and arterial pressure. There were no significant changes in metabolic values, or propofol serum concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Substance P increases cochlear blood flow without changing cochlear electrophysiology in rats

Carotid artery infusions of substance P yielded reductions in systemic blood pressure and elevations in cochlear blood flow (CoBF), measured via laser Doppler flowmeter, with no alterations in cochlear action potentials or cochlear microphonics in Wistar-Kyoto rats. Additionally, direct micro-infusions of substance P into the anterior inferior cerebellar artery, which contributes to the local vascular perfusion of the cochlea, yielded elevations in CoBF with no changes in systemic blood pressure. Pretreatment with a specific substance P receptor antagonist, ([D-Pro2,D-Trp7,9]SP) via the carotid artery or the anterior inferior cerebellar artery, diminished subsequent substance P-induced vascular responses. These results suggest that endogenous substance P, like other vasoactive peptides, may interact with a substance P-specific receptor population in the cochlea and may therefore participate in the ongoing regulation of CoBF. These findings also support the premise that vasodilatory hormones, along with vasoconstrictive agents, may be involved in the autoregulation of CoBF.

Effects of stellate ganglion stimulation on bilateral cochlear blood flow

The effect of intraneural electrical stimulation of the stellate ganglion (SG) on bilateral cochlear blood flow (CBF) was investigated with laser-Doppler flowmetry. The SG of 15 anesthetized guinea pigs was exposed by a novel surgical approach and stimulated with a specially designed intraneural bipolar platinum-iridium electrode. Bilateral CBF was continuously monitored. Stimulation of 0.25 mA caused a detectable increase of the systemic blood pressure (BP) and a bilateral decrease of the cochlear vascular conductance (R, defined as the ratio CBF/BP). A stimulus of 0.5 mA elicited a statistically significant ipsilateral CBF (CBFi) decrease of 3.6% +/- 5.1% from the baseline and a contralateral CBF (CBFc) decrease of 3.1% +/- 5.5%. That no statistical difference was found between CBFi and CBFc indicates that a unilateral sympathetic stimulation of the SG can cause equal bilateral responses. These responses were accompanied by a significantly increased BP (8.7% +/- 5.2% of baseline) and consequently a greatly decreased R (12.2% +/- 6.5%) of the ipsilateral cochlea. Bilateral sections of the cervical sympathetic trunk below the level of the superior cervical ganglion did not alter the evoked changes in CBF, BP, and R. It is concluded that SG stimulation can decrease the conductivity of the cochlear vessels or the supplying vessels of the cochlea. Additionally, the SG nerve fibers that cause these effects do not pass through the superior cervical ganglion.

The interaction of smoking and noise on temporary threshold shifts

Four separate experiments examined the effects of cigarette smoking on temporary threshold shifts (TTS) following noise exposure. One experiment compared smokers and non-smokers after the subjects had abstained from smoking for at least 6 h. A second experiment tested only smokers who smoked a cigarette just before noise exposure, during the noise exposure and in a control condition during which they did not smoke. A third experiment tested only non-smokers who were exposed to noise after chewing nicotine gum or after a control condition in which they rested without chewing the gum. A fourth experiment tested smokers and non-smokers in one condition which required them to smoke a cigarette just prior to noise exposure and in another condition which prevented them from smoking. The results indicate that smokers consistently evidence slightly smaller TTS than do non-smokers. Non-smokers evidenced significantly greater TTS at one frequency after they had chewed nicotine gum than when they had not. These results suggest that the smaller TTS associated with cigarette smoking is related to both the chronic and the acute effects of smoking and that these effects may be more attributable to carbon monoxide than to nicotine.

Cochlear blood flow and microvascular resistance changes in response to hypertonic glycerol, urea, and mannitol infusions

The effect of hyperosmotic agents on cochlear blood flow (CBF) was tested in normal guinea pigs and in guinea pigs having prior unilateral operations to ablate the endolymphatic duct. Laser-Doppler-measured CBF was normalized to remove apparent changes related directly to systemic blood pressure. Hyperosmotic fluids were given via venous infusion: glycerol (20% and 40% solutions), urea (10%, 30%, and 40% solutions), and mannitol (40% solution). All agents were dissolved in 0.9% saline and the mixtures were given at a rate of 0.3 to 0.6 mL/min for 5 minutes. Control infusions were of 0.9% saline and isotonic dextran 70 (Pharmacia). All hyperosmotic infusions resulted in similar increases in normalized cochlear blood flow (nCBF) that extended to a maximum of 300% of the baseline value in a dose-dependent way during the infusion time period. Within approximately 30 minutes following infusions, nCBF had returned to baseline levels. Saline infusion alone had little effect on nCBF, but isotonic dextran 70 gave a sustained increase to 122% of the baseline levels. There was no difference between the responses of nCBF in hydropic and normal cochleas for either control or hyperosmotic solutions. Measurements of systemic hematocrit at time intervals during and following the infusions showed that transient reductions of up to approximately 8% (for the maximum osmotic challenge) occurred during the infusion. It is concluded that the hyperosmotic treatments tested here are equally effective for short-term enhancements of nCBF in both normal and hydropic cochleas. The basis of the flow increase is partially rheologic and partially due to a local vasodilation.

Effects of hydroxyethyl starch, nimodipine, and propylene glycol on cochlear blood flow

A primary goal of pharmacologic treatment for otopathologies of vascular origin is to elevate cochlear blood flow (CoBF), thus facilitating the transport of oxygen and nutrients without compromising perfusion pressure in other tissues. In the present investigation, significant increases in CoBF were measured during intra-arterial infusion of the plasma expanding agent, hydroxyethyline starch (HES), and the vasodilator nimodipine, in anesthetized adult male guinea pigs. There were no changes in systemic blood pressure during the infusion of HES or nimodipine. Intra-arterial infusion of propylene glycol (PG), which is used as a nonaqueous solvent, produced inconsistent CoBF effects accompanied by initial decreases in systemic blood pressure with subsequent increases. It is concluded that nimodipine and HES are very promising agents for inducing increases in CoBF, whereas PG produced inconsistent effects on CoBF while elevating blood pressure, thus compromising its potential usefulness in the treatment of otopathologies.

The influence of intra-arterial infusion of arginine vasopressin on cochlear blood flow in the rat

Intra-arterially infused arginine vasopressin (AVP) elevated systemic blood pressure (BP) in the Sprague-Dawley rat according to a dose-response pattern while cochlear blood flow (CoBF), as measured by laser Doppler flowmetry, was elevated only at the highest dose. Skin blood flow (SBF) decreased significantly with AVP infusion. The local infusion of AVP into the anterior inferior cerebellar artery (AICA), which supplies the common cochlear artery, produced significant dose-dependent reductions in CoBF with no changes in systemic blood pressure. Pretreatment of the local cochlear supplying vessels with an AVP-specific V1 receptor antagonist attenuated subsequent AVP-induced decreases in CoBF, thereby demonstrating specificity of the response. These results suggest that CoBF is reasonably stable in response to systemic AVP infusion until blood pressure exceeds an elevation from base level of approximately +60 mm Hg. One of the mechanisms responsible for this autoregulatory response may be vasoconstriction mediated by the interaction of vasoactive peptides such as AVP and its receptors located in the vasculature of the inner ear or in the more peripheral vessels directly supplying the cochlea.

Measurement of human cochlear blood flow

Cochlear blood flow (CBF) was measured with a laser-Doppler (L-D) flowmeter (Periflux PR2-B) in four unanesthetized human subjects with chronic tympanic membrane perforations and nine anesthetized human subjects undergoing middle ear operations. The L-D recordings were made over the promontory and/or the round window membrane during carbogen breathing and direct electrical stimulation of the cochlea in both groups and with warm water irrigation of the external ear canal in the anesthetized subjects. Carbogen led to little or no change in CBF as monitored with either measurement approach in either subject group. Electrical stimulation yielded an increase (15% to 25%) in CBF as recorded from the promontory in seven of the nine subjects tested. Warm (44 degrees C to 49 degrees C) water irrigation produced changes of 20% to 60% in CBF that were partially recoverable in the 10 minutes available for study. This study demonstrated the feasibility of direct CBF measurement in humans with the L-D method. Moreover, the data indicate that carbogen has little influence on CBF and that electrical stimulation at relatively safe levels and warm water irrigation of the ear canal produce increases in human CBF.

Effects of perilymph volume adjustments on cochlear blood flow in the guinea pig

Previous research suggests a potential relationship between perilymphatic pressure (Pp) and cochlear blood flow (CBF); however, the alterations in Pp necessary to produce changes in CBF have not been adequately described or quantified. The effects of perilymph volume changes on systemic blood pressure (BP) and CBF were presently investigated in the guinea pig cochlea. Five microliters of perilymph were displaced in each of three conditions: viz. evacuation of 5 microliters from the cochlea; replacement of these 5 microliters; and finally the addition of 5 microliters of artificial perilymph into the cochlea. All perilymph volume adjustments were completed in 1-microliter increments during which changes in CBG and BP were recorded. Significant alterations in CBF were observed during 1-microliter perilymph volume adjustments in each condition with no significant changes in systemic BP. The results from this study support our hypothesis that an inverse relationship exists between CBF and Pp in that decreases in perilymph volume yielded elevations in CBF while increases in perilymph volume yielded reductions in CBF.

Relationship between cochlear blood flow and perilymphatic oxygen tension

To clarify the characteristics of the blood circulation in the cochlea, we correlated cochlear blood flow and perilymphatic oxygen tension at various blood pressures. Cochlear blood flow was measured in guinea pigs by laser Doppler flowmetry, and perilymphatic oxygen tension by polarography. Blood pressure changes were induced by angiotensin II injection, trimetaphan camsylate injection and blood withdrawal. Cochlear blood flow generally paralleled systemic blood pressure, indicating a close correlation. In contrast, perilymphatic oxygen tension was slower to increase and decrease. However, when systemic blood pressure was lowered more gradually, perilymphatic oxygen tension did not show the same lag. These findings indicate that perilymphatic oxygen tension parallels systemic blood pressure when changes induced are slower and in a physiological range.

Androgenic effects on angiotensin II-induced blood pressure and cochlear blood flow changes in rats

Ovariectomized, castrated, and sham-castrated rats pretreated with oil or testosterone were intra-arterially infused with saline and three doses of angiotensin II while blood pressure and cochlear blood flow were measured. The results indicated a positive dose-response relationship for blood pressure and cochlear blood flow. Sham-castrated males had higher mean blood pressure responses than castrated males, followed by ovariectomized females. Cochlear blood flow responses were higher in the sham-castrated males than the ovariectomized females, followed by the castrated males. In comparison to the male groups, the ovariectomized females evidenced the lowest, middle, and highest cochlear blood flow responses to the three increasing doses of angiotensin II. Testosterone pretreatment facilitated angiotensin-induced cochlear blood flow elevations in all three angiotensin doses. These results suggest that endogenous and exogenous androgens may alter blood pressure and/or cochlear blood flow responses to angiotensin II via different mechanisms.

Pentoxifylline increases cochlear blood flow while decreasing blood pressure in guinea pigs

The effects of pentoxifylline on cochlear blood flow (CoBF) were investigated in anesthetized guinea pigs by laser Doppler flowmetry and intravital microscopy red blood cell velocity measurement. Intra-arterial infusion of pentoxifylline (3, 4, and 5 mg/kg/min) produced dose-dependent reductions in blood pressure, accompanied by significant elevations in CoBF that were not dose-dependent. These results are in general agreement with previous findings from our laboratory utilizing normotensive and spontaneously hypertensive rats, however, in contrast with rats, guinea pigs revealed an initial decrease in CoBF followed by an increase. Also, pentoxifylline produced relatively smaller elevations in CoBF in guinea pigs as compared with those previously reported in rats. Taken together these results support the hypothesis that pentoxifylline increases vascular perfusion by decreasing blood viscosity and increasing the plasticity of red blood cells.

Mannitol and dextran increase cochlear blood flow in normotensive and spontaneously hypertensive rats

Pharmacologic treatments for otopathologies of possible vascular origin are aimed at enhancing vascular transport and are intended to increase cochlear blood flow (CBF). In the current study, normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHRs) were given intravascular infusions of mannitol (20%), dextran 70 (6%), or control saline. In general, the results clearly indicate that both osmotic agents effectively increase CBF, as measured by laser Doppler flowmetry. However, the mechanism of action of these agents is less clear.

Laser light transmission and laser Doppler blood flow measurements on the human, rat and guinea pig cochlea

In order to test the applicability of laser-Doppler flowmetry in monitoring cochlear blood flow clinically, the thickness and the helium-neon laser light transmission of specimens of human, rat and guinea pig promontory bone and human skin were determined. Furthermore, comparative laser-Doppler measurements were taken from the promontory in patients, rats and guinea pigs. Due to the different thicknesses of the promontory bone in different species, the light transmission was found to be considerably higher for the animal cochlea (rat, 15%; guinea pig, 6.6%) than the human cochlea (1.7%). However, a clearly higher laser-Doppler signal was recorded from both the human and the rat cochleas as compared with the guinea pig. The relative laser light attenuation by the human skin specimens corresponded to that of the human promontory bone. The findings are discussed with regard to the suitability of the laser-Doppler method for blood flow measurements in the human cochlea.

Measurement of cochlear blood flow: intravital fluorescence microscopy

A technique is described for directly observing in vivo cochlear microvasculature in the gerbil for physiologic and experimentally induced changes in vessel diameter and blood flow velocity. Measurements are made from computer processed video images of surgically exposed microvessels. These images are obtained using intravital fluorescence microscopy (IFM) with epi-illumination. The Mongolian gerbil is an ideal animal model for circulatory studies of the inner ear. It has a stable heart rate and blood pressure under urethane/alpha-chloralose anesthesia and its cochlea is surgically accessible. A window is created over the feeding artery (anterior inferior cerebellar artery) and over the stria vascularis of the second turn of the cochlea, atraumatically exposing radiating arterioles and strial capillaries. Our system of IFM provides images that are videorecorded, digitally analyzed with a computer image processor, and enhanced according to the type of measurement desired. Velocity measurements are obtained by tracking plasma gaps or single fluorescent labeled red blood cells through successive frames of the videorecorded images. This experimental technique allows us to analyze circulatory responsiveness to a variety of vasoactive drugs administered regionally to the cochlea in concentrations not affecting systemic circulation.

A comparison of laser Doppler and intravital microscopic measures of cochlear blood flow

Many inner ear disorders may be caused by alterations in cochlear blood flow (CBF). However, each measurement technique used to monitor CBF has limitations in examining the relationship between otopathologic states and blood flow. This study investigates laser Doppler flowmetry (LDF) and its fundamental drawback: the unknown relationship of LDF output to actual CBF. LDF readings are directly compared with concurrent intravital microscopy (IVM) measures of erythrocyte velocity in the lateral wall of the guinea pig cochlea. Positive end expiratory pressure, spontaneous respiration of 5% and 10% carbon dioxide, phenylephrine, and direct electrical stimulation of the cochlea were used to manipulate CBF. High, positive correlations were found between simultaneous LDF and IVM measurements of CBF. In addition, the study demonstrated that current microdissection techniques used to perform IVM do not cause changes in CBF. IVM measurements of CBF are a more sensitive indicator of CBF changes than are LDF measures. Despite the high correlation between measurement techniques within a single manipulation, simultaneous LDF and IVM measurements differed between manipulations. This may reflect regional changes in CBF affected by these manipulations and differences in the sampled vascular beds contributing to these two measures. It is unlikely that a single calibration factor can be defined that would allow the conversion of LDF output to actual units of blood flow across different manipulations used to alter CBF.

The effects of intracerebroventricularly administered angiotensin II on blood pressure and cochlear blood flow in rats and guinea pigs

Guinea pigs and Sprague-Dawley rats were intracerebroventricularly (icv) infused with various doses of angiotensin II (AII) in order to investigate central control of and species differences in cochlear blood flow (CBF) and blood pressure (BP). The results indicated a positive dose-response relationship between icv infusions of AII for BP and CBF in members of both species. This relationship was more predictable in the guinea pig than in the rat. In addition, responses to similar AII doses between species indicated that: a) BP elevations were significantly greater in the rat than in the guinea pig, b) CBF elevations were not significantly different between the rats and guinea pigs, and c) CBF was more highly correlated with BP in the guinea pig than in the rat. These results suggest that different relationships may exist between BP and CBF in rats and guinea pigs.

Cochlear blood flow autoregulation in Wistar-Kyoto rats

Wistar-Kyoto rats (WKY) were intra-arterially infused with angiotensin II (AII) or phenylephrine for 10 min. Both vasoactive compounds produced an initial increase in cochlear blood flow (CoBF) as measured by laser Doppler flowmetry, followed by a slow steady return to baseline, despite sustained elevations in systemic blood pressure. These results suggest autoregulation of CoBF in the WKY rat. In a second experiment. All was infused directly into the anterior inferior cerebellar artery (AICA) which feeds the cochlear artery. Significant reductions in CoBF were noted without changes in systemic blood pressure. Pretreatment with the specific angiotensin-receptor antagonist, sarthran (Sar1, Thr8-AII), diminished subsequent AII-induced reductions in CoBF. These results indicate that AII binding to vascular receptors may induce vasoconstriction in the supplying vessels of the cochlea, and thus, the interaction of blood-borne AII and vascular angiotensin receptors may participate in the autoregulation of CoBF.

Blood pressure and cochlear blood flow in the guinea pig

The relationship between blood pressure and cochlear blood flow was investigated in 68 guinea pigs, using the vasoactive drugs angiotensin II, norepinephrine, phentolamine, isoproterenol, dobutamine, salbutamol, propranolol, bradykinin, papaverine, vinpocetine dilazep, and brovincamine. Cochlear blood flow increases markedly and proportionately to increases in blood pressure. By contrast, cochlear blood flow shows various responses toward a fall in blood pressure. In general, cochlear blood flow appears relatively resistant to blood pressure decrease.

Autoregulation of cochlear blood flow in normotensive and spontaneously hypertensive rats following intracerebroventricularly mediated adjustment of blood pressure

Previous studies in our laboratory (Quirk et al., 1988) noted significantly impaired elevations in cochlear blood flow (CoBF) during systemic infusion of the potent vasoconstrictive agent angiotensin II (AII) in the spontaneously hypertensive rat (SHR) as compared with the normotensive Wistar-Kyoto (WKY) rat, despite similar increases in systemic blood pressure. We interpreted these results to suggest that SHRs have an exaggerated autoregulatory mechanism that controls blood supply to the cochlear vessels. However, an alternative explanation for these findings concerns the potential influence of the elevated baseline blood pressure of the SHR on CoBF. Specifically, if there is an absolute threshold blood pressure that triggers an autoregulatory response in the cochlea, then the SHRs would reach that threshold sooner than normotensive animals because they begin at a baseline blood pressure that is well above that of the WKY rat. The present study addressed this possibility by pharmacologically reducing SHR blood pressure to WKY baseline blood pressure and raising WKY to SHR baselines, followed by the infusion of previously utilized doses of AII. The results are consistent with previous findings and support our interpretation of an exaggerated autoregulation of cochlear blood supplying the SHR.

The influence of carbogen on susceptibility to TTS

A group of 20 male subjects were exposed to noise in each of four separate conditions. In two conditions they inhaled carbogen (95% oxygen, 5% carbon-dioxide). In two others they inhaled compressed air. In two conditions they were exposed to noise after inhalation and in two they were exposed to noise during the inhalation. The results indicated that carbogen inhalation reduced the magnitude of TTS but only in the 'During' condition.

Cochlear blood flow: measurement techniques

The measurement of inner ear blood flow and other microvascular variables is subject to unique technical problems which are compounded by methodological limitations. As a result, the interpretation of experimental results is often difficult. This report discusses the most important methods currently available for cochlear blood circulation measurements and the technical problems associated with their use. The use of a combination of measurements to resolve problems of interpretation is stressed. An extensive review of the pertinent literature is provided in relation to each method.

The effects of pentoxifylline on cochlear blood flow in normotensive and spontaneously hypertensive rats

Normotensive and spontaneously hypertensive rats (SHRs) were given arterial infusions of saline and pentoxifylline in doses of 1.5, 2.0, and 3.0 mg/kg/min. Blood pressure decreased in a dose related fashion with increasing doses of pentoxifylline. Cochlear blood flow, measured via laser Doppler flowmeter, increased with infusion of pentoxifylline in members of both rat strains, but to a greater extent in SHRs than in normotensive animals. The mechanism of action appears to be decreased red blood cell rigidity which allows increased penetration of red blood cells into microvessels of the cochlea.

Observations on cochlear blood flow using the laser Doppler method in guinea pigs

Cochlear blood flow of the guinea pig was measured using the laser Doppler method. Blood pressure and respiratory rhythm were also recorded simultaneously. The reliability of this method was substantiated by comparing it with the hydrogen clearance method using rabbit auricles. The blood flow of the auricle was altered artificially by clipping these vessels and then simultaneously measuring the blood flow by the two methods (Doppler and hydrogen clearance). The data obtained by these two methods were closely correlated. Norepinephrine induced both an elevation of blood pressure and an increase in cochlear blood flow in the guinea pig. Phentolamine, on the other hand, induced a fall in blood pressure and a slight decrease in cochlear blood flow. In addition, phentolamine completely blocked the effects of norepinephrine on blood pressure and cochlear blood flow. In this experiment, the responses of blood pressure and cochlear blood flow induced by norepinephrine and phentolamine were dose-related.

Cochlear blood flow increases after systemic hemodilution: comparison of simultaneous laser Doppler flowmetry and radioactive microsphere measurements

Guinea pig cochlear blood flow was measured before and after systemic normovolemic hemodilution with high molecular weight dextran. Absolute determinations of blood flow (in the cochlea, brain, kidney and lung) were accomplished by use of radioactive-labeled (85Sr or 141Ce) microspheres. Relative measurements of the cochlear blood flow changes were made simultaneously by the use of a laser Doppler flowmeter. The flowmeter probe was placed on the first cochlear turn. Hemodilution to an average systemic hematocrit of 20% increased cochlear blood flow by 250% as measured with microspheres. The laser Doppler instrument significantly underestimated the actual flow increase giving an indication of 148%. Furthermore, the data, when analyzed on an individual trial basis, showed a very poor correlation between the two methods. The theoretical basis for these findings in relation to the use of the laser Doppler instrument is discussed.

Blood pressure in resting, anesthetized and noise-exposed guinea pigs

This study investigated blood pressure in guinea pigs while they were 1) alert and free moving, 2) anesthetized with different anesthetics, and 3) exposed to continuous, 115 dB SPL white noise under anesthesia. The animals were prepared with a carotid artery catheter and permitted to recover for 48 h before blood pressure levels were measured. Mean arterial blood pressure in the resting, unrestrained guinea pig was 64 mmHg (+/- 1.38 S.E.). Ketamine Hydrochloride (Ketamine) significantly decreased, and Fentanyl-Citrate significantly increased, blood pressure. Fentanyl-Droperidol produced no substantial blood pressure change. Guinea pigs anesthetized with Fentanyl-Citrate and Fentanyl-Droperidol demonstrated significant blood pressure increases when exposed to noise, with the Fentanyl-Citrate group showing a greater response. Animals anesthetized with Ketamine Hydrochloride exhibited no significant blood pressure changes when exposed to the noise.

Angiotensin II-induced changes in cochlear blood flow and blood pressure in normotensive and spontaneously hypertensive rats

Previous investigations in our laboratory have measured significant increases in the circulating levels of the potent vasoconstrictive hormone, angiotensin II (AII; 26 and 64 pg/100 microliters plasma, normal and noise exposed, respectively), during and following noise exposure in the alert rat (Wright et al., 1981). In the present study, these levels were approximated through intra-arterial infusion in the anesthetized spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat. Laser Doppler flowmeter measurements of cochlear blood flow (CBF) indicated that despite equivalent AII-induced elevations in systemic blood pressure, CBF in the SHR did not increase to the levels measured in the WKY. Pretreatment with the specific angiotensin receptor antagonist sarile, (Sar1,Ile8-AII), reduced AII-induced elevations in systemic blood pressure in members of both strains, but did not change the overall pattern of CBF. These results indicate that SHRs may have a compromised cochlear circulation that is refractory to increases in systemic blood pressure.

Recent advances in cochlear blood flow measurements

Changes in blood flow to the inner ear have been thought to influence or underlie a number of cochlear diseases, including some forms of noise-induced hearing loss, sudden hearing loss, and Meniere's disease. Recently, important advances have been made in two technologies for the study of cochlear blood flow. The first is in the area of vital microscopic studies of cochlear microcirculation, and the second is based on the introduction of laser technology in the form of laser Doppler flowmetry. In this report, measurements are given of changes in cochlear circulation caused by carbon dioxide breathing, intravenous phenylephrine injection, systemic hemodilution, positive end expiratory pressure, and direct electrical stimulation of the cochlea. From these changes, we observe that cochlear blood circulation responds to systemic blood pressure alterations and is subject to local flow control mechanisms. Linearity and speed of response of the laser Doppler instrumentation also are shown. These advances show promise for contributing to our knowledge of control mechanisms of inner ear blood flow and for revealing the influence of various pharmacologic agents of potential clinical value.

Histological measures of cochlear blood flow. A validation study

Cochlear blood flow in guinea pigs was assessed using histological assessment of various red blood cell and vessel lumen parameters. Cochlear blood flow was either enhanced by administration of carbogen or inhibited by induced positive airway pressure. The resulting enhancement or reduction of blood flow in the cochlea was verified by laser Doppler velocimetry. The results generally indicate that the two measures of cochlear blood flow agree. This agreement was especially apparent when histological parameters of the vessel lumen were considered and when external wall vessels were evaluated.

Effects of carbon monoxide on cochlear electrophysiology and blood flow

The belief that the cochlea is particularly vulnerable to a reduction in oxygen availability comes predominantly from studies reporting the disruption of electrophysiological measures, such as the compound action potential, endocochlear potential, inner hair cell intracellular potentials or afferent nerve fiber responses by asphyxiation. Because hypoxia has frequently been suggested as an underlying mechanism by which many ototoxic agents produce injury, and because such agents are not likely to completely disrupt oxygen delivery, we investigated the effects of graded hypoxia (using doses of carbon monoxide) on cochlear blood flow, the compound action potential (CAP) and the cochlear microphonic (CM). High doses of carbon monoxide injected intra-peritoneally yielded reversible loss of the CAP sensitivity for high frequency tone bursts, the extent of which was dose dependent. The loss was observed first at the highest frequency tested (50 kHz) and as carboxyhemoglobin levels increased, contiguous lower frequencies were influenced. Recovery progressed from low to high frequencies as carboxyhemoglobin levels declined. Carbon monoxide administration also produced a dose dependent elevation in the cochlear blood flow measured by a laser Doppler flow monitor. The data suggest that carbon monoxide administration disrupts cochlear function only under extremely severe exposure conditions. An elevation in cochlear blood flow may well serve as a protective mechanism which maintains cochlear function in the face of declining blood oxygen carrying capacity and delivery. While the site of action of carbon monoxide in the cochlea is uncertain, the data clearly indicate that elements involved in the generation of the CAP for high frequency tones are particularly vulnerable.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hemodilution on cochlear blood flow measured by laser-Doppler flowmetry

The effect of hemodilution on cochlear blood flow was studied in guinea pigs. Hypervolemic hemodilution was accomplished by infusion of 10 mg/kg of body weight of dextran 40 (as a 10% solution in normal saline), which resulted in an average hematocrit decrease from 43 to 32%. Normovolemic hemodilution was accomplished by repeated exchange of 3 ml of whole blood with 3 ml of dextran 75 (6% solution in normal saline) every 5 minutes until the hematocrit reached approximately 5%. The cochlear blood flow was measured by laser-Doppler flowmetry. Irrespective of the dilutional technique, the cochlear blood flow increased as hematocrit decreased to a maximum of approximately 200% of original value at a hematocrit near 20%. The blood pressure was not significantly influenced by the hemodilution until hematocrit values below 15% were reached. The enhancement of cochlear blood flow is consistent with the expected reduction of blood viscosity and increase of cardiac output. Normovolemic hemodilution with dextran 75 causes a smaller disturbance of systemic circulation physiology and has a more lasting effect than dextran 40 infusion.

Techniques for the observation and measurement of red blood cell velocity in vessels of the guinea pig cochlea

Fluorescence techniques combined with intravital microscopy provide a powerful approach to the study of cochlear blood microcirculation. In the current study, fluorescein isothiocyanate conjugated to high molecular weight dextrans was added to plasma to enhance the visual contrast of flowing blood in microscopic images from the guinea pig cochlea. Photometric signals, obtained from video pictures of the blood vessels, provided a means to continuously measure red cell velocity by using crosscorrelation algorithms to extract the time delay for moving features of the image. Alternatively, a small amount of fluorescently-labeled red blood cells (RBCs) were added to the vascular volume to serve as natural indicators of whole blood flow. The speed of these cells was measured by video photometric detection of the time required for the cells to pass between two predetermined positions in the television image. RBCs can be made fluorescent by chemical bonding of a fluorochrome to the cell membrane or by internal loading of the cell with an inert fluorochrome. Labeled RBCs provide a means to determine blood velocity in capillaries having extremely poor optical contrast, a situation which is generally the case for relatively thick tissues such as the lateral wall of the membranous labyrinth.

Laser Doppler measurements of cochlear blood flow during loud sound exposure in the guinea pig

This investigation examined the effects of loud sound of different frequencies and intensities on cochlear blood flow as measured by the laser Doppler flowmeter. Cochlear blood flow was measured in anesthetized guinea pigs during a 1 h exposure to either a 2, 4, or 12 kHz pure tone or high-pass noise (10-40 kHz) at 90, 103, or 110 dB SPL. Cochlear function was assessed using the compound action potential audiogram before and after exposure. There was no change in blood flow in the second turn with a 2, 4, or 12 kHz tone but there was a significant (P less than 0.05) decline in flow in the first cochlear turn at the end of either the 12 kHz tone or high-pass noise exposure at 103 and 110 dB SPL. There were elevations in the thresholds of the cochlear compound action potential after all but the 90 dB exposures to 12 kHz or high-pass noise. No such changes were observed in blood flow or electrophysiology in control animals. These findings demonstrate that there is a small but significant decline in cochlear blood flow with high intensity sound exposure. However, the relationship between this change in blood flow and the development of cochlear damage is unclear.